U.S. fighter jets will patrol the airspace over Prague, and the Czech government is mobilizing 12,000 police officers, 2,200 heavily armed soldiers and special anti-terrorist units to protect President Bush and four dozen other heads of state.

Al-Qaida and other terror groups are not the only worry for organizers of the two-day summit, which begins Thursday: Thousands of militant protesters have vowed to converge on Prague and stage demonstrations that authorities fear could get out of hand.

The Czechs, still mopping up after August floods that swamped parts of the medieval capital, are anxious to avoid a repeat of September 2000, when opponents of economic globalization laid siege to a summit of the International Monetary Fund and the World Bank.

Hooded protesters dug up the city's ancient cobblestones and hurled them and Molotov cocktails at police, who responded with tear gas, clubs and water cannons. More than 600 people were injured and the scenic capital was trashed.

Police last week arrested several suspects they alleged were plotting to cut power lines to the meeting venue and Prague's subway system. Border authorities also have intercepted and turned away a number of known hard-core protesters on Interpol lists, Vokus said.

Black Bloc

Among the most militant groups planning 50 separate actions this week is one known as the Black Bloc, whose members wear black masks and advocate violence as the only effective means of protest.

The Black Bloc was blamed for instigating much of the violence and vandalism that marred a July 2001 summit of the Group of Eight in Genoa, Italy, where one protester was killed and more than 200 others injured.

The Czech Anarchist Federation also has threatened to stage 34 demonstrations between Tuesday and Friday, including some at the Prague Congress Center, where the summit will be held, and at the city's medieval castle, which Bush and other heads of state plan to visit.

In a call to action on its Web site, the group said: "Let's turn Prague into Buenos Aires!" -- a reference to bloody demonstrations in the Argentine capital last December in which 27 protesters were killed.

Authorities said they knew of plans by several other shadowy groups, including Anti-Fascist Action, the AntiNATO Coalition and the Organization of Revolutionary Anarchists. Up to 35 more moderate leftist groups and communist parties from across Europe plan lower-key protests, the Interior Ministry said.

Miroslav Grebenicek, chairman of the Czech Communist Party, is among those put off by the heavy security. In an open letter to Interior Minister Stanislav Gross last month, Grebenicek complained it has "criminalized" the expected demonstrations and "intimidated participants," who have a right to protest NATO policies.

Up to 250 U.S. troops also will be deployed around Prague, and 15 American military aircraft -- including F-15 and F-16 fighters and a reconnaissance plane equipped with AWACS radar -- have been authorized to secure the skies.

Outside help is needed because the Czech Republic, a NATO member since 1999, cannot adequately protect its entire airspace. The air force has just a few older supersonic Soviet-made MiG-21 fighters and several subsonic L-159s.

Czech police planned to seal off roads near the summit venue and the castle, post snipers on rooftops, and tighten security at border checkpoints and around hotels and foreign embassies.

NATO is expected to admit up to seven former Soviet bloc countries at its first summit behind the former Iron Curtain. Leaders also plan to approve a makeover of the military alliance to create elite rapid-response units capable of more efficient anti-terrorist missions.